The present invention relates to an optical communication network system, and more particularly to an optical communication network system for performing switching of routes of an optical signal without converting the optical signal into an electric signal.
In order to deal with a rapid increase in data traffic represented by the Internet and a sudden increase in demand for multimedia communications including images, audio and data, a higher speed operation and a larger capacity operation have been pushed ahead for a transmission line and a communication node, which constitute a communication network, and there have been progresses made in the introduction of an optical communication apparatus using an optical fiber and an optical signal. In addition, in place of a conventional communication apparatus for processing an optical signal, in which the optical signal is converted into an electric signal once, studies have been conducted on practical use of an optical cross-connect apparatus (OXC) and an optical add-drop multiplexing apparatus (OADM) for performing switching process such as switching of transmission routes/signal lines without converting an optical signal into an electric signal. The OXC and the OADM uses an optical switch as a main component for switching optical transmission lines. As an optical switch, various types have been known, e.g., a mechanical optical switch, an optical switch using a thermooptical effect, an optical switch using an electrooptical effect and the like. Among these types, the mechanical optical switch is most often used because a power loss thereof is the smallest.
For the practical use of the OXC or the OADM, it is essential to provide an apparatus, which is configured to improve basic performance such as suppression of a power loss of an optical signal or the like and to be capable of properly switching and operating signal routes (or a system itself), and to be excellent in reliability, availability and serviceability (hereinafter referred to as “RAS function”). In a conventional transmitter or a digital switching device such as a multiplexer processing an electric signal, performance of a signal to be processed has been monitored in a proper position, or a redundant configuration (e.g., duplication) in a part of an apparatus has been adopted. Thus, an apparatus having an excellent RAS function has been provided.
In the conventional case of using the electric signal, time multiplexing can be carried out up to 10 Gbps and, in principle, routes can be switched by using this technology. However, in a digital transmission system such as SONET/SDH, to execute process of high-level control management signal, 64 signals of 155 Mbps are arrayed in parallel development, and routes are switched. For such a speed, a technology for switching by using a data buffering technology without any momentary power failures power-interruption has been known.
As described above, the OXC and the OADM uses the optical switch as the main component for switching routes of an optical signal. However, in the OXC and the OADM directly processing the optical signal, there occurs a problem, in the case of the most often used mechanical optical switch, that a switching speed is slow, which is several milli-seconds at the shortest, while a transmission rate of the optical signal to be passed is 10 Gbps or higher (e.g., 40 Gbps), which is much higher than that of an electric signal. Consequently, if switching of signal routes similar to the conventional apparatus for processing an electric signal is simply executed for the OXC or the OADM, a momentary power failure occurs, where an optical signal of several million bits, that is, several tens of frames, is lost because of its inability to pass through the optical switch during optical signal route switching by the optical switch. In other words, the momentary power failure that has been prevented by the conventional apparatus for processing the electric signal occurs in the OXC or the OADM directly processing the optical signal. Thus, a need arises to realize an optical signal switching apparatus having an excellent RAS function on the assumption of presence of a momentary power failure by an optical switch.
Generally, in the OXC or the OADM, in order to maintain performance of an optical signal to be processed, after switching of optical routes, various factors are monitored, which include (1) optical signal power deterioration/failure [detection level: −20 dBm, detection time: order of 1 μsec.], (2) synchronous state of an operation clock [detection time: order of 1 μsec.], (3) synchronous state of an optical signal frame [detection time: 375 μsec,], (4) optical signal error rate (bit error rate, referred to as BER, hereinafter) [detection level: 10·9, detection time: 10 sec.], and the like. This monitoring is carried out for a predetermined time, and optical signal route (or system itself) is properly switched to another when a trouble or a possibility of a trouble is discovered. Such a trouble monitoring function is essential for an improvement of the RAS function. The detection levels and the detection times, which are bracketed in the above-described factors, are only examples, and can be properly changed depending on a speed of an optical signal to be processed by the apparatus or a size or installing place of the apparatus.
In the apparatus provided with the above-described trouble monitoring function, depending on an installing position of a monitoring circuit or a monitoring method, a momentary power failure due to route switching by the optical switch may be detected as an optical signal power failure, BER degradation or stepping-out of synchronization. Consequently, even if the switching is a normal operation, a situation may occur where an alarm is given to the downstream side of an optical signal advancing direction or an apparatus for monitoring and controlling troubles. In addition, generally, the monitoring circuit also verifies a normal state after completion of the route switching or monitors recovery from the trouble. Thus, unless monitoring is carried out by considering time necessary for route switching by the optical switch or an operation time of the above-described trouble monitoring function, even if the switching has been normally carried out, a situation may occur where an alarm is given to the downstream side of the optical signal advancing direction or the apparatus for monitoring and controlling troubles. In the OXC or the OADM, such a situation induces repeating route switching even if an operation is normal. Consequently, an operation of the entire OXC or OADM, or an operation of a communication system (network) using the OXC or the OADM becomes unstable, it brings about a state for the RAS function can not be operated as desired. Needless to say, such a situation can be prevented by introducing a protective function for extending trouble detection time, recovery monitoring time and the like. However, such a method is not preferable for an improvement of the RAS function because an original alarm monitoring ability is reduced.
Meanwhile, in the conventional communication apparatus for processing an electric signal, such as a digital switching device and the like, the one has been known, which is configured to previously mask erroneous information caused by an in-apparatus operation (e.g., system switching, hardware maintenance/switching) based on software instruction in order to prevent collection thereof and then to carry out an operation, to collect by the software an alarm or management information monitored by hardware in the apparatus, and the like. However, the masking function by the software in the conventional communication apparatus for processing an electric signal cannot simply be applied to the OXC or the OADM.
As a specific example, when an optical route is switched by the optical switch, a momentary power failure causes an optical signal power failure, and stepping-out of clock synchronization (hereinafter referred to as clock stepping-out) and stepping-out of frame synchronization (hereinafter referred to as frame stepping-out) of a transmission signal. However, recovery from the optical signal power failure is detected during switching time (about 1 milli-sec.) after completion of switching. Meanwhile, for the clock stepping-out and the frame stepping-out, after optical signal power is recovered by a new connection, new clock and frame synchronization must be performed. Time necessary for verifying re-synchronization exceeds 1 milli-sec. Further, 10000 frames are necessary for BER measurement since frame synchronization is secured, and the process must wait for 10 sec. Consequently, when correct operation of route switching is carried out by distinguishing a momentary power failure due to switching of the optical switch from a disconnection of an optical fiber as a fixed trouble, if only the conventional trouble detection method or the conventional masking function by the software simply is applied to the OXC or the OADM, the RAS function becomes short. Thus, there is a demand for an OXC or an OADM having an excellent RAS function for detecting a real trouble and switching routes in consideration of a combination of a plurality of factors for trouble detection and monitoring time thereof with a trouble detection/recovery detection operation carried out following disposition of a trouble detection circuit in an apparatus. Furthermore, there is a demand for an OXC or an OADM preventing notification of an alarm to a downstream side of an optical signal advancing direction or an apparatus for monitoring and controlling a trouble even if a momentary power failure occurs due to route switching, and preventing induced re-switching of routes while an operation is normal.